Plural reflector antenna with polarization rotation to minimize feedshadow



3 4: :5 6 U U I i QLHHUH HUVI Feb. 25, 1969 G. LE-PARQUIER ETAL 3,

PLURAL REFLECTOR ANTENNA WITH POLARIZATION ROTATION TO MINIMIZE FEED SHADOW Filed March 25, 1966 United States Patent US. Cl. 343-756 Int. Cl. H0111 19/14, 15/22 5 Claims ABSTRACT OF THE DISCLOSURE A Cassegrain antenna having a feed system to minimize shadow effect. A centrally mounted feed faces a polarization selective first reflector which reflects all the waves to a polarization rotating second split reflector. The second reflector rotates the polarization of the waves and directs them back to the first reflector which is now transparent to the new polarization.

The present invention relates to antenna systems with two reflectors, for example of the Cassegrain type. More particularly, the invention relates to antennas wherein the polarization direction is not the same for the incident and the reflected signal.

In an antenna system comprising a reflector, the primary source casts a shadow on the reflecting surface. This adversely affects the antenna gain and increases the secondary lobes. This can be avoided at least in part by offsetting the primary source but the counterpart is that is that a dissymmetry is created.

It is an object of the invention to provide an antenna system wherein the shadow effect is avoided and yet the primary source remains in a central position.

According to the invention there is provided an antenna system having an axis of symmetry comprising: a primary source located on said axis, for radiating waves having a given direction of polarization; a first reflector, positioned on the path of said wave, which is not transparent for the waves having said given direction of polarization and which is transparent for the waves having a different direction of polarization, said first reflector being symmetrical with respect to said axis; a second reflector positioned on the path of the waves reflected by said first reflector and which modifies the direction of polarization of waves incident thereon, said second reflector being symmetrical With respect to said axis; and a dioptric system, symmetrical with respect to said axis and positioned on the path of the waves reflected by said second reflector, said dioptric system making parallel the waves incident thereon.

According to a preferred embodiment of the invention, the two reflectors build up an aplanatic system.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawing accompanying the following description and the only figure of which shows by way of example one embodiment of the system according to the invention.

The system shown comprises a primary source 1, or an array of primary sources, normal to the plane of the fig- 3,430,246 Patented Feb. 25, 1969 ure; a first reflector which is not transparent for the polarization direction of the source 1 and which is formed by two identical paraboloid portions which have the same focus and are symmetrical with respect to the axis of the antenna system; a second reflector 3 formed by two plane reflector elements, also symmetrical with respect to the axis of the antenna system, and a double prism 4. The second reflector is so built that it modifies the polarization direction of the incident signal. Preferably, the second reflector builds up, together with the first reflector, an aplanatic system.

The spherical wave which is generated by the primary source 1 is reflected by reflector 2 as two plane waves which propagate along the respective axes of the two paraboloids building up the reflector 2.

These two plane waves are respectively reflected, while their polarization direction is modified, by the two plane elements 3 and then propagate through reflector 2, which is transparent, for the polarization direction they have by now, towards the double prism 4 which receives the two incident waves and changes their respective propagation directions to make them parallel, so that there is, at the output of the prism, a single plane wave propagating along the axis of the antenna system.

It should be noted that all the elements building up the system of the invention are entirely conventional and therefore need not be described in more detail.

Thus, the polarization rotating reflector may comprise, as is well known, a reflecting metal surface, covered with a layer of dielectric, such as a resin, in which i incorporated a mesh of parallel wires which are spaced by at most half the maximum operating wavelength and inclined by 45 with respect to the polarization of the incident wave, all this being well known in the art.

The antenna system of the invention has an exit aperture without any-shadow or inactive surface portion, thus improving the antenna performance as to gain, secondary lobes and diffused radiations.

Also the standing wave ratio is improved since the waves reflected by reflector 2 do not interfere with the primary source.

Of course the invention is not limited to the embodiment described and shown, since obviously the essential component parts of the antenna system according to the invention, namely the semitransparent reflector system, the reflector system which causes the polarization direction to change and the dioptric system used, may be of various types without departing from the spirit and scope of the invention.

What is claimed is:

1. An antenna system having an axis of symmetry comprising: a primary source located on said axis for radiating waves having a given direction of polarization; a first reflector positioned on the path of said wave which is not transparent for the waves having said given direction of polarization and which is transparent for the waves having a different direction of polarization, said first reflector comprising two parts symmetrical with respect to said axis, the two parts of said reflector being portions of respective surfaces intersecting each other on said axis, the planes respectively tangent to said surfaces on said axes being at an angle with respect to each other; a second reflector positioned on the path of the waves reflected by said first reflector and which modifies the direction of polarization of waves incident thereon, said second reflector being symmetrical with respect to said axis; and a dioptric system symmetrical with respect to said axis and positioned on the path of the waves reflected by said second reflector, said dioptric system making parallel the waves incident thereon.

2. A system according to claim 1, wherein said different direction is perpendicular to said given direction.

3. A system according to claim 2, wherein said parts are adjacent surface portions, symmetrical with respect to said axis.

4. A system according to claim 2, wherein said first reflector is formed by two adjacent identical paraboloid portions which have the same focus and are symmetrical respect to said axis, the axes of said paraboloids being at an angle with respect to each other and said second re- 15 4' flector is formed by two adjacent plane reflector elements, also symmetrical with respect to said axis, the planes of said elements being at an angle with respect to each other. 5. A system according to claim 2, wherein said first and second reflectors build up an aplanatic system.

References Cited UNITED STATES PATENTS 2,736,895 2/1956 Cochrane 343-756 ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 343-781, 837, 914 

